When I started the online game just now I went to the world map and selected an army I had located on my city. When I selected the transfer option there are no units showing up in my city at all that can be transferred to the army.

Yes there are units in my city when I look at the city map and yes my army has space.

I've tried restarting the game and leaving/returning to the city.

If I move a unit from my army to the city it does show up and I can transfer it back again.